The effects of hemorrhagic shock on the hearts of dogs will be determined. Measurements will be made before, during and after periods of oligemic hypotension of myocardial blood flow (using radio-labelled microspheres), cardiac output, and coronary arterio-venous levels of O2 and lactate. Myocardial lesions will be studied by light and electron microscopy. These findings will be compared with those made in dogs that are, in addition, treated with dipyridamole or propranolol.